1. Technical Field
The invention pertains generally to a lithographic process for producing devices such as semiconductor devices, and more particularly to a method involving resists.
2. Art Background
Lithographic processes play an important role in the manufacture of devices such as semiconductor devices. During the manufacture of these devices lithographic processes are used to pattern substrates, such as silicon wafers or processed silicon wafers which are, for example, wholly or partially covered by metal, silicon dioxide, or polysilicon. That is, a substrate is coated with an energy sensitive material called a resist. Selected portions of the resist are exposed to a form of energy which induces a change in the solubility of the exposed portions in relation to a given developing agent or etchant. The more soluble portions of the resist are removed and portions of the substrate are bared by applying the developing agent or etchant to the resist. The bared portions of the substrate are then treated, e.g., are etched or metallized.
Both organic and inorganic materials have been used in resists for the patterning of substrates. Exemplary inorganic materials are chalcogenide glass-based materials, i.e., materials exhibiting a noncrystalline structure and whose major constituent is sulfur, selenium, or tellurium, or combinations thereof. Included among the chalcogenide glass-based materials used as resists are germanium-selenium (Ge/Se) glass films supporting relatively thin layers of silver selenide (Ag.sub.2 Se). Typically, the Ge/Se films are evaporated or rf-sputtered onto the surface of a substrate. A thin layer of Ag.sub.2 Se is formed on the surface of a Ge/Se film by, for example, dipping the film into a [Ag(CN).sub.2 ].sup.- -containing aqueous solution, the resulting chemical reactions with the film yielding the Ag.sub.2 Se. (See R. G. Vadimsky, K. L. Tai, Abstract No. 318, 158th Electrochemical Society Meeting, Hollywood, Fla., Oct. 5-10, 1980.)
When a Ag.sub.2 Se-covered Ge/Se film is exposed to an appropriate form of energy, silver ions from the Ag.sub.2 Se layer migrate into the exposed regions of the Ge/Se glass film, decreasing the solubility of these regions to specific developers. (See e.g., K. L. Tai, L. F. Johnson, D. W. Murphy, M. S. C. Chung, Abstract No. 94, Vol. 79-1, p. 244, Electrochemical Society Meeting, Princeton, N.J., 1979, regarding photo-induced migration of silver into germanium-selenium). Silver migration into Ge/Se glasses is induced, for example, by UV light (in the wavelength range from about 2000 to about 4500 Angstroms), low energy electron beams (having energies ranging from about 1 kev to about 3 kev), and low energy ion beams (including ions such as helium, nitrogen, argon, xenon, and gallium ions with energies ranging from about 10 kev to about 30 kev). (See A. Wagner, D. Barr, T. Venkatesan, W. S. Crane, V. E. Lamberti, K. L. Tai, R. G. Vadimsky, Journal of Vacuum Science Technology, 19(4), Nov./Dec. 1981, regarding the use of low energy ion beams).
After a Ge/Se film is exposed to energy, the Ag.sub.2 Se remaining on the surface of the film is removed. This is done, for example, by immersion in a KI/KI.sub.3 solution which converts Ag.sub.2 Se to AgI and dissolves the latter by forming KAgI.sub.2. Thereafter, the Ge/Se film is either dry developed, or more conveniently wet developed by dissolving the nonexposed Ge/Se in a strongly alkaline (pH of about 12.5) aqueous solution, e.g., a sodium hydroxide solution. Sodium sulfide has been added to developers used to develop Ge/Se films containing 90 atomic percent Se, in order to assist in the removal of the relatively large amount of Se in these films. (See R. G. Vadimsky, K. L. Tai, Abstract No. 318, 158th Electromechanical Society Meeting, Hollywood, Fla., Oct. 5-10, 1980 regarding wet development of Ge/Se resists).
One of the advantages associated with germanium-selenium inorganic resists is their high absorbance of various forms of energy, including UV light, low-energy electron beams, and low-energy ion beams. That is, at least 60 percent of these energies, when incident on germanium-selenium resists, is absorbed within a thin image layer about 100 to 300 Angstroms thick. Consequently, if a pattern of lines and spaces is delineated in a germanium-selenium inorganic resist, the variation in the desired widths of the lines and spaces will be relatively small (less than or equal to about 10 percent), as compared to most organic resists. That is, good linewidth control is achieved with germanium-selenium resists because little or no energy penetrates beyond the 100 to 300 Angstrom-thick image layer, and thus reflections from the substrate supporting the germanium-selenium resist, with their attendant degradation in linewidth control, are avoided.
Another advantage associated with germanium-selenium inorganic resists is the ability of these resists to resolve feature sizes smaller than 1 .mu.m while achieving excellent linewidth control (variations in linewidth less than or equal to about 10 percent). It is believed that this is due to the so-called edge-sharpening effect. (Tai et al, Abstract No. 318, 158th Electrochemical Society Meeting, Hollywood, Fla., Oct. 5-10, 1980, and Tai et al, "Submicron optical lithography using an inorganic resist/polymer bilevel scheme," Journal of Vacuum Science Technology, 17 (5), Sept./Oct. 1980, pp. 1169-1175, have explained this desirable effect).
Germanium-selenium resists are often used in combination with relatively thick organic polymers. The thick organic polymers are used to planarize stepped surfaces, i.e., to present a flat surface to the germanium-selenium resists so that their high resolution capabilities can be exploited. In addition, the thick organic polymers are used as masks for dry etching substrates.
When a germanium-selenium resist is used in combination with a thick organic polymer, the polymer, having a thickness of about 2.mu., is first spun onto the substrate. Then, a relatively thin film of germanium-selenium, having a thickness of about 2000 Angstroms, is evaporated or rf-sputtered onto the thick organic polymer. After a pattern is defined in the germanium-selenium film, i.e., after the germanium-selenium film is exposed and developed, the pattern is readily transferred to the organic polymer by reactive ion etching which etches the polymer but not the germanium-selenium film. Generally, the germanium-selenium film is removed and the etched organic polymer then serves as a mask for patterning the substrate.
As discussed above, Ge/Se glass resists have desirable attributes. However, improvement in linewidth control is always desirable.